The invention relates to a control system and method for matching the stimulation frequency of a heat pacemaker to the varying load condition of a patient.
In a control system such as disclosed for example in U.S. Pat. No. 4,399,820 issued Aug. 23, 1983, a measurable variable S.sub.O2 corresponding to the central venous blood oxygen saturation in the heart is identified according to the principle of reflection oximetry, and a controlled variable is derived therefrom in that changes of this measurable variable over a given time increment are standardized to the maximum change of the measurable variable over long time intervals such as hours or days.
In a control system of this type, there is further provided a processing means for storing the measurable variable S.sub.O2 as ascertained in successive short time intervals, and also storing limit values S.sub.O2max and S.sub.O2min which are respectively the maximum and minimum values of the measurable variable as ascertained over a relatively long time interval. From these maximum and minimum values, a maximum change of the measurable variable is obtained, which may be designated by the notation .DELTA.S.sub.O2max. A first controlled variable is then obtained by forming the quotient of a change in the measurable variable, designated .DELTA.S.sub.O2, and the maximum change .DELTA.S.sub.O2max. In this way, the change in the measurable variable over a short time interval is standardized on the basis of the maximum change of the measurable variable over a relatively long time interval.
A long-term, undisturbed measured value acquisition and a good haemodynamic situation in the blood circulation are already obtainable with the control system of U.S. Pat. No. 4,399,820. In this system, so called follower control is effected such that the frequency is modified by a constant amount .DELTA.f whenever the controlled variable exceeds a prescribable value. In the control system of the U.S. patent, an optimizing control is superimposed on the follower control, the optimizing control becoming effective when no frequency modification has been produced by the follower control over a prescribable time interval.
The known control system uses chronological changes of the standardized measurable variable as control signal. The consequence of this is that the frequency changes are time-dependent and the stimulation frequency changes dependent on the duration of the change in the blood oxygen saturation. Identical overall changes of the blood oxygen saturation can therefore lead to different changes of the heart frequency depending on how quickly they occur. In unfavorable cases, this type of follower control can lead to the result that the follower control does not achieve values that are physiologically optimally matched. These non-optimum control values are then not compensated until the subsequent optimizing control. Under certain conditions, however, this type of successive follower and optimizing control can lead to unstable conditions, whereby the heart rate oscillates between the prescribed limit values as a consequence of periodically fluctuating load conditions. It has also been shown that the chronological separation of the two control events, i.e. the separation of the follower control and of the optimizing control by a substantial intervening time interval, can prevent optimizing of the control action when short-duration changes of the blood oxygen saturation are present which are superimposed on a steady state and elicit a frequency change via the follower control.
In addition to this control method of U.S. Pat. No. 4,399,820, wherein chronological changes of the measurable variable are used as controlled variable, U. S. Pat. No. 4,202,339 discloses a method wherein the frequency matching of the heart pacemaker is effected according to the relationship EQU f.sub.p =k.multidot.S.sub.O2, f.sub.min &lt;f.sub.p &lt;f.sub.max
that is wherein the heart pacemaker frequency f.sub.p is varied according to the respectively identified measured value of the blood oxygen saturation itself. The values for k, f.sub.min and f.sub.max are in this case permanently selected before the implantation. This control method exhibits a number of problems: Thus, no changes can be allowed to appear on the optical transmission link. Deposits on the measuring probe or foreign objects in the reflection region (cardiac wall, trabecula) also lead to falsifications of the measured results. Further, this control principle requires a calibration before implantation. A considerable improvement of this situation could already be achieved if a programmable heart pacemaker which could also supply intracardial measurable variables to an external monitoring and control station in dialogue were employed. A new calibration and, thus, a new determination of the characteristic could thus be undertaken under given conditions at regular intervals or as needed. The outlay required therefor, however, is undesirably high.